In general, you adjust normals and smoothing to prepare objects for rendering. A normal is a unit vector that defines which way a face or vertex is pointing. Smoothing groups define whether a surface is rendered with sharp edges or smooth surfaces.
The direction in which a normal points represents the front, or outer surface of a face or vertex, which is the side of the surface that is normally displayed and rendered. You can manually flip or unify face normals to fix surface errors caused by modeling operations or by importing meshes from other programs.
Smoothing groups are numbers assigned to the faces of an object. Each face can carry any number of smoothing groups, up to the maximum of 32. If two faces share an edge and share the same smoothing group, they render as a smooth surface. If they don’t share the same smoothing group, the edge between them renders as a corner. You can change and animate smoothing group assignments manually. Changing smoothing groups does not alter geometry in any way: it simply changes the way faces and edges are shaded.
When you create an object, normals are generated automatically. Usually objects render correctly using these default normals. Sometimes, however, you need to adjust the normals.
Smoothing blends the shading at the edges between faces to produce the appearance of a smooth, curved surface. You can control how smoothing is applied to a surface so your objects can have both smooth surfaces and sharp, faceted edges where appropriate.
